1. Field of the Invention
The present invention relates to an apparatus and method for transmitting/receiving an ACKnowledgement/Negative ACKnowledgement (ACK/NACK) signal for received packet data in order to support Hybrid Automatic Repeat reQuest (HARQ) in a Frequency Division Multiple Access (FDMA) wireless communication system.
2. Description of the Related Art
With reference to FIG. 1, FDMA will first be described below. FIG. 1 illustrates FDMA.
Referring to FIG. 1, FDMA is a technology for distinguishing physical channels in frequency. In general, all available resources are divided in time and frequency as indicated by reference numeral 101. A minimum block is composed of one symbol in time and one subcarrier in frequency. This is called a Time-Frequency (TF) bin. A TF bin is an actual transmission unit carrying a modulation symbol on a physical channel. The total number of TF-bins depends on a total frequency bandwidth and the number of symbols transmittable in a Transmission Time interval (TTI).
In FDMA, different TF bins are allocated to different channels and different User Equipments (UEs). Basically, it is impossible to share one TF bin between different channels or different UEs. However, to achieve time diversity, frequency diversity, or spatial diversity for a low-rate channel, a TF bin can be shared by covering or spreading the TF bin with a code as with Code Division Multiplexing (CDM). For transmission of high-rate packet data, TF bins are purely allocated. Mapping between data symbols and TF bins is equivalent to subcarrier mapping 102. The mapping relationship between a channel and TF bins is signaled beforehand or determined according to a predefined rule.
Since a channel carrying packet data or a channel carrying signaling data is allocated on a UE basis, TF bins are allocated to the channel and then the channel with TF bins is allocated to an intended UE. Although TF bins can be allocated on a UE basis, TF bins allocated to a specific channel form a logical channel as indicated by reference numeral 103, which is preferable in terms of signaling. One logical channel (hereinafter, “channel”) is composed of a plurality of TF bins and the number of TF bins is determined, taking into account the characteristics of the channel. In determining the number of TF bins for a channel, the lowest data rate of packet data is considered if the channel is a packet data channel, and signaling overhead of information about a scheduled channel is considered if scheduling is carried out. For a control channel, the number of TF bins is determined according to the number of bits transmitted per TTI.
A channel to be allocated to a UE is scheduled every TTI or set by higher signaling. In the illustrated case of FIG. 1, if a Data CHannel (DCH) 103 is allocated to a UE, packet data symbols 105 for the UE are mapped to the DCH 103 by channel mapping 104 and then mapped to actual TF bins by subcarrier mapping 102. During the subcarrier mapping 102, the channel may be mapped to scattered TF bins (e.g. a DCH 106) or successive TF bins (e.g. the DCH 103), depending on whether frequency diversity is to be achieved or according to a TF bin 4 allocation algorithm. Since the channel is a logical channel, when only an allocated channel is transmitted as with a UE, there may not be a need for channel mapping because transmission symbols are simply mapped to predefined TF bins of the allocated channel.
HARQ is a technique for increasing a reception success rate by soft-combining initial transmission data with retransmission data without discarding the initial transmission data. A HARQ receiver determines whether a received packet has errors and transmits a HARQ ACK signal or a HARQ NACK signal to a HARQ transmitter according to the determination result. Accordingly, the HARQ transmitter retransmits the HARQ packet or transmits a new HARQ packet according to the received HARQ ACK/NACK signal.
HARQ is categorized into synchronous HARQ and asynchronous HARQ according to the timing of retransmission. In synchronous HARQ, a retransmission occurs a predetermined time after completion of a previous transmission, whereas a retransmission occurs irrespective of the time of a previous transmission in asynchronous HARQ.
With reference to FIG. 2, a synchronous HARQ operation will be described in more detail. FIG. 2 illustrates a basic HARQ operation.
Referring to FIG. 2, a HARQ transmitter transmits an initial HARQ packet on a DCH 202 by a predetermined process in step 203. A HARQ receiver decodes the initial HARQ packet and determines whether the initial HARQ packet has errors by a Cyclic Redundancy Check (CRC) check. In the presence of errors, the HARQ receiver stores the HARQ packet in a buffer and transmits an HARQ NACK to the HARQ transmitter on an ACK CHannel (ACKCH) 202 in step 205. In step 206, the HARQ transmitter retransmits the HARQ packet. The HARQ receiver soft-combines the stored HARQ packet with the retransmission HARQ packet and performs a CRC check in step 207. If the combined HARQ packet still has errors, the HARQ receiver stores the HARQ packet in the buffer and transmits an HARQ NACK to the HARQ transmitter. However, if decoding of the combined HARQ packet is successful, the HARQ receiver transmits an HARQ ACK to the HARQ transmitter in step 208.
The HARQ transmitter repeats the above operation until it receives an HARQ ACK from the HARQ receiver or the number of retransmissions for the HARQ packet reaches a predetermined retransmission number.
Now an ACK/NACK transmission method will be described.
Conventionally, a dedicated channel is allocated for a UE so that the UE can transmit an ACK/NACK signal. Under an environment where channels are non-orthogonal as with Code Division Multiple Access (CDMA), the total amount of available resources is limited by transmit power or reception interference level rather than it is directly related to the number of codes. Therefore, allocation of a code to each UE is not a significant problem in terms of resource utilization even if the UE does not use the dedicated channel. However, T-F resources are orthogonal and the amount of T-F resources directly affects that of available resources in FDM. Hence, when T-F resources allocated to an ACKCH are not used, it is a waste of resources. In other words, dedicated allocation of resources for ACK/NACK transmission on a UE-by-UE basis is inefficient in terms of resource utilization in an FDMA system.
In this context, one-to-one mapping between ACKCHs and DCHs or Shared Control CHannels (SCCHs) has been proposed and is under discussion in order to support HARQ efficiently in the FDMA system.
FIG. 3 illustrates one-to-one mapping between DCHs and ACKCHs.
Referring to FIG. 3, reference numerals 302 to 305 are DCHs and reference numerals 307 to 310 denote ACKCHs. The DCHs 302 to 305 are mapped to the ACKCHs 307 to 310 in a one-to-one correspondence and an ACK/NACK signal for a received DCH is transmitted on a predetermined ACKCH mapped to the DCH. If packet data is received on a first DCH 302 (DCH #1), an ACK/NACK signal for the packet data is transmitted on a first ACKCH 307 (ACKCH #1). If packet data is received on a second DCH 303 (DCH #2), an ACK/NACK signal for the packet data is transmitted on a second ACKCH 308 (ACKCH #2). The mapping between the ACKCHs and the DCHs enables ACK/NACK transmission without allocating dedicated frequent resources to UEs.
FIG. 18 illustrates one-to-one mapping between SCCHs and ACKCHs.
Referring to FIG. 18, reference numerals 1802 to 1805 are SCCHs and reference numerals 1807 to 1810 denote ACKCHs. The SCCHs 1802 to 1805 are mapped to the ACKCHs 1807 to 1810 in a one-to-one correspondence and an ACK/NACK signal for a received DCH is transmitted on a predetermined ACKCH mapped to an SCCH by which the DCH has been scheduled. If scheduling information about packet data is received on a first SCCH 1802 (SCCH #1), an ACK/NACK signal for the packet data is transmitted on a first ACKCH 1807 (ACKCH #1). If scheduling information about packet data is received on a second SCCH 1803 (SCCH #2), an ACK/NACK signal for the packet data is transmitted on a second ACKCH 1808 (ACKCH #2).
ACK/NACK repetition will be described below.
Typically, an ACK/NACK TTI is equal in length to a TTI of a general downlink frame or an uplink frame. When a Mobile Station (MS) at a cell boundary needs a transmit power exceeding a maximum allowed power, for ACK/NACK transmission, it transmits an ACK/NACK signal with the maximum allowed power. The resulting decreased received signal level renders the ACK/NACK transmission unreliable. To avert this problem, a High Speed Downlink Packet Access (HSDPA) system repeats the same ACK/NACK signal, so that instantaneous power level requirements are decreased as much as a repetition number and thus the ACK/NACK signal can be transmitted within a maximum allowed power level. Information about whether an ACK/NACK signal is repeated (hereinafter, referred to as ACK/NACK repetition setting information) is set in an upper-layer signaling message or a Medium Access Control (MAC) message by a network.
For ACK/NACK repetition, a repetition factor should be set in order to indicate whether an ACK/NACK signal is repeated and how many times the repetition occurs. For example, if the repetition factor is a non-zero number, the ACK/NACK signal is repeated as many times as the repetition factor. If the repetition factor is 0, the ACK/NACK signal is transmitted only once.
When a system with ACKCHs mapped to DCHs or SCCHs supports ACK/NACK repetition, it faces some problems, which will be addressed with reference to FIG. 4. In the illustrated case of FIG. 4, one cell has two UEs, UE #1 and UE #2. UE #1 is located at a cell boundary and UE #2 is near to a Node B at the center of the cell.
First, one-to-one mapping between DCHs and ACKCHs will be described.
Referring to FIG. 4, three ACK/NACK repetitions are set for UE #1 so that it can transmit an ACK/NACK signal reliably. Since UE #2 has sufficient transmit power, UE #2 is supposed to transmit an ACK/NACK signal only once.
Upon receipt of packet data on a first DCH 402 (DCH #1) in step 405, UE #1 transmits an ACK/NACK signal on a first ACKCH 402 (ACKCH #1) at time k=4 in step 407 and then repeats the ACK/NACK signals on ACKCH #1 at time k=5 and k=6 in steps 408 and 409. Meanwhile, a Node B may transmit packet data to UE #2 on DCH #1 during the next TTI through scheduling in step 406. Then UE #2 transmits an ACK/NACK signal for the received packet data on ACKCH #1 at time k=5 in step 410. Thus, the ACK/NACK signals from UE #1 and UE #2 collide on ACKCH #1 at time k=5. This data collision occurs because UEs share the ACKCHs and the DCHs are mapped to the ACKCHs in a one-to-one correspondence.
ACK/NACK repetition is viable on the premise that an ACKCH can be allocated to a UE for a plurality of TTIs. However, data is transmitted on a DCH only during one TTI and the one-to-one mapping between ACKCHs and DCHs does not allow for allocation of an ACKCH mapped to the DCH long enough for ACK/NACK transmission. As SCCHs are also transmitted on a TTI basis, the one-to-one mapping between SCCHs and ACKCHs illustrated in FIG. 18 leads to the same problem.